1. Field of the Invention
The present invention relates generally to oil field equipment, and more specifically to an apparatus suitable for use in removing dense liquids and sediment from the bottom of an oil storage tank.
2. Description of the Prior Art
Oil which is removed from land based wells is often pumped into tanks for temporary storage. The oil is removed periodically from the storage tanks for transport to a central location by tank trucks. Depending on the flow rate of the well or wells connected to the storage tank, oil can remain in the storage tank for as long as two to three weeks.
Because of various sediments suspended in the recovered oil, and denser liquids mixed with the oil, settling occurs in the storage tanks. The heavier liquids and sediments settle to the bottom of the tank, where they tend to form discrete layers graded by density. Typically, salt water and paraffin form two fairly discrete layers underneath the oil.
The settled denser liquids, such as salt water and paraffin, are pumped out of the storage tank at intervals. Removal of this unwanted sludge at the bottom of the storage tank allows more oil to be stored into the tank, decreasing the frequency with which the storage tank must be emptied. Also, when the oil is eventually removed, its overall quality is better when these impurities have been removed beforehand.
FIG. 1 illustrates a technique currently used to perform oil storage tank cleanout. A storage tank 10 is shown partially filled with oil 12. A sludge containing paraffin 14 underlines the oil 12. The top of the storage tank 16 contains a small opening 18, commonly referred to as a thief hatch, which allows access to the interior of the tank 10. A loading valve 20 positioned about 12 inches from the bottom of the tank 10, allows oil to be removed from the tank 10 when it is full.
From time to time the paraffin 14 is removed from the tank 10. This may be done on a regular basis, with the frequency of removal being determined by the pumping rates of the attached well or wells (not shown) and the known composition of the oil stored in the tank 10. Alternatively, a periodic check can be made of the paraffin level within the tank 10. A common technique for performing the check involves sampling the fluid on the tank bottom using a device known as a thief (not shown) which is dropped through the thief hatch 18. Another technique uses a metal tape (not shown) coated with a chemical which changes color in the presence of water. Dropping such a tape through the thief hatch 18 allows an estimate to be made of the amount of paraffin residing in the tank 10. Whatever technique is used, when the paraffin level becomes too deep, a service call can be made and a worker dispatched to clean out the storage tank 10.
Cleanout of the storage tank 10 is performed by lowering a riser hose 22 through the thief hatch 18. Its lower end has a fitting 24 which prevents the opening of the riser hose 22 from becoming blocked by the floor of the storage tank 10. A pump hose 26 is connected to the riser 22, and connects to a fluid pump attached to a tank truck (not shown).
Sludge is removed from the bottom of the storage tank 10, a process commonly referred to as "pulling the bottom" of the tank. This process involves having a worker attach the pump hose 26 to the riser 22, and lower the riser through the thief hatch 18. The worker then returns to the tank truck and starts the pump which begins pumping fluid from the bottom of the tank 10. After some period of time which seems appropriate to the worker, a sample is taken of the liquid currently being pumped from the tank 10. If good oil shows, a sufficient amount of paraffin is assumed to have been removed from the tank, and the hoses can be removed. This is accomplished by ascending to the top of the storage tank and pulling the riser hose 22 out through the thief hatch 18 while the pump is operating. This causes the liquid currently in the riser hose 22 and pump hose 26 to be cleared and emptied into the tank truck, allowing the hoses to be more easily stored on the tank truck for transport to another location.
The pumps used on the tank trucks for this operation transfer liquid at a typical rate of approximately three barrels per minute. It will be appreciated that, due to the delay in removing the riser 22 from the tank 10, several barrels of good oil will be pumped out of the tank. Additionally, several barrels of good oil are often pumped into the tank truck before a sample is taken to determine that enough paraffin has been pumped out of the tank. This results in good oil being wasted, and increases disposal fees because an excess volume of liquid is removed from the tank each time it is cleaned out.
In addition to pumping out excess amounts of good oil, FIG. 1 illustrates another problem with the technique described above. Salt water layers on the bottom of the tank tend to be removed almost completely, inasmuch as the salt water has a relatively low viscosity and a hole does not tend to form in such layer. However, the paraffin 14 has a relatively high viscosity and a hole will tend to be formed in the paraffin layer 14 in the immediate neighborhood of the riser hose 22 as shown. This means that good oil can be pumped out of the tank while there is still a significant amount of paraffin remaining in it. This often causes the worker to complete the cleanout operation while there is a significant amount of paraffin remaining in the tank. Once the paraffin layer settles down, a later sampling may still show an excess depth of paraffin. This means that the cleanout must be repeated.
Thus, in general, the prior art technique for pulling the bottom of an oil storage tank removes too much good oil and results in too much paraffin remaining behind. This results in increased production costs due to the repeat trips which must be made to clean out the storage tank and to the loss of good oil. Additional costs are incurred if a production hauler comes to empty the tank, and refuses the oil because of too much paraffin contain therein. Also, refusal of a tank can necessitate shutting down the associated wells if the tank is full. All of these problems add to the cost and decrease the profits to be made from the well.
It would be desirable to provide an apparatus and method for removing denser liquids from the bottom of an oil storage tank which maximizes the amount of paraffin and other liquids removed and minimizes the amount of good oil removed.